After starting the level, kill all the guards. Now look at the picture on the easel. It shows you the layout of that section of the post office. The red X is you, the green arrow is the fork lift, the green Xs are enemies, and the red O with a line through it is the place with the smoke grenade.
At the start of the level, kill the first two SWAT men. Then, travel about halfway down and you should see a whiteboard. Go up to it and look at it in first person view. It shows the plan for the level, where they will attack from and how many SWAT men are in every section.